Known processes for separating and classifying particles contained in an aggregate particle mass are truly numerous. Many of these processes are limited to separating particles according to size (classifying) as, for example, the traditional and common screening and size-separating processes. Other processes are effective in separating particles in accordance with their weight or shape. The present invention relies heavily on differences in density among the classified particles in a dry fluid bed, and can be used in combination with other types of separation techniques.
One of the oldest methods for separating heavier materials from lighter crushed materials is the riffle board, or riffle pan, in which crushed ore is placed upon a corrugated surface set at an incline and flushed with water. During separation, the riffle board is moved back and forth in directions normal to the corrugations, or is otherwise vibrated so as to create relative motion between the particles and the riffled surface. The ligher ore tends to carry over the corrugations (riffles) farther from the point of feed than the heavier minerals, and the crushed materials therefore are carried by the water over the edge of the riffle board at different locations.
A disadvantage in the riffle board separation process is its requirement for the continuous flow of fluid over the riffles. In addition, the riffles are necessarily restricted in dimension and thus a limit is placed upon the amount of material which can be contained by any riffle, and thereby upon the amount of material which may be separated in a given amount of time.
Another technique for upgrading crushed ore particles is found in U.S. Pat. No. 3,349,904. There a rotating screen in the form of an inverted cone receives the aggregate particle mass while air is simultaneously blown up through the screen to create an upward pressure. Heavier metal particles are intended to overcome the upward air blast pressure and be separated out of the mass by falling through the screen, while lighter rock particles are thrown upwardly and outwardly to the periphery of the screen due to centrifugal force. One major disadvantage in attempting to separate particles by this method is the high degree of complexity of the apparatus and the requirement for a pressurized air source. An obvious limitation is that material sized larger than the screen openings, even if having the selected density, cannot be handled. Furthermore, although it may be possible to separate particles whose densities are grossly disparate, it is believed that particle size must be very carefully controlled where the density of the desired material (such as crushed ore) approaches the density of the waste material.
Particle handling equipment may use a gyratory separator (or "classifier") such as that disclosed in U.S. Pat. No. 2,950,819. A particle mixture is placed upon a vibratory screen designed to pass particles of all sizes smaller than the screen openings and irrespective of the particles' densities. Separators of this type are usually operated to cause all over-size particles to move to the periphery of the screen to be discharged. It is possible, however, to operate such devices so that oversize particles do not discharge due to a tendency for them to move radially inwardly to the center of the screen where they are retained as is shown, for example, in U.S. Pat. No. 3,794,165 (FIGS. 7-10). In certain cases, these separators are used to remove or recover particles entrained in liquid, the latter flowing through the screen and leaving behind particles trapped by the vibratory screen. These particles are flushed down an outlet at the screen's center.
In all cases, so far as is known, gyratory separators have not been adapted to or operated for separating particles in accordance with the relative densities. Even in cases where particles are retained on the vibratory screen, no provision was made for separately segregating or extracting those remaining particles according to their densities.
Still another known separation technique is based upon a mechanical concentrator known as the Denver mechanical concentrating pan which duplicates the miner's hand-panning motion. This device consists of a series of classifying screens under which are placed several pans specially coated to trap the fine heavy materials (e.g., gold). The first pan is metal-coated with mercury to amalgamate free gold; the remaining pans receive the overflow from the first and are coated with a rubber matting covered with screening which acts like a riffle. The entire assembly is driven with an eccentric motion in order to swirl the material in water, which is added along with the particle mixture to settle the mineral. Like other processes, this technique requires a flow of water and its collection capacity of the heavier fines is limited by the amalgamation and riffle capacity of the concentrating pans. It thus must be stopped periodically and emptied of the concentrated materials.
A similar principle is used in devices such as shown in U.S. Pat. No. 1,141,972 to Muhleman, where a rotary tilting motion is imparted to a pan having a riffled floor surface. Concentrated ore is extracted from a hole in the center of the pan floor. Again, the motion of the pan is such that the waste material swirls about the edge of the pan and is discharged, whereas heavier material gravitates toward the center due to the tilting.
In my U.S. Pat. No. 4,148,725, I disclose a new process and apparatus wherein a size-classified bed of particles is fluidized by agitating a supporting surface with a gyratory motion to fluidize the particle bed. Particles are contacted with vertically projecting annular surfaces movable with the supporting surface and defining two or more annular channels. Particles within these regions are given sufficient fluidity by their reaction against the surfaces to allow them to move within the particle bed and distribute themselves according to their relative densities. Thus, particles move from one channel to the next through restricted openings in the annular surface, the denser particles tending to accumulate in one channel and the less dense particles being displaced into adjacent channels. The dense particles migrate in the direction of the eccentric "throw" toward a collection zone. Their greater energy, or momentum, it is thought, is what causes them to remain in the collection zone and displace less dense particles there.
In accordance with the teachings of said U.S. Pat. No. 4,148,725, particles may be added to the fluidized particle bed at one of the interior annular channels so that waste material (e.g., less dense particles) flows outwardly. The denser particles, on the other hand, tend to be given a net inward momentum where they collect at the central collection region.
The foregoing process and apparatus functions effectively when recovering dense particles from a bed of coarse sand when operating on either a continuous basis or a "batch" basis. In processing by batch, a fixed amount of feed material is loaded into the apparatus, which then is operated for a given period of time and shut down. Thereupon the densified, or upgraded, concentrate is extracted. It was found, however, that a problem sometimes arose when attempting to separate dense particles from fine sand, e.g., sand finer than -30 mesh. In such case, the sand tends to become compacted in the collection zone to such a degree that dense particles sought to be recovered cannot move through the compacted mass and, accordingly, may not reach the collection zone.
There are, perhaps, many reasons for the above phenomenon; however, the interrelated motion of the particles and the reaction surfaces is so complex that a dispositive analysis cannot be readily made. Because the net force on all particles tends to drive them toward the collection zone, it has been observed that particles tend to be more compacted at the center of the bed. Another explanation may be that the vertical displacement of the gyratory motion at the center of the bed is at a minimum, the maximum vertical displacement occurring at the bed periphery. Thus, it is possible that the fluidity of particles moving toward the center of the bed diminishes excessively due to a reduction in this vertical displacement.
Another problem that has been experienced is the limitation in the flow rate through the gyratory apparatus. When the equipment was operated in a continuous flow mode, high flow rates had to be avoided in order that the dense particles not be carried out of the bed with the less dense waste material. Separation times, therefore, could be much longer than is ordinarily acceptable for some commercial operations. On the other hand, if the equipment were operated on a batch basis to avoid inadvertent loss of dense particles, through-put is reduced. This is because particles have to be given sufficient residence time in order to penetrate the particle bed and collect in the collection zone. The amount of particles that can be batch processed at any one time is limited to the bed capacity, as the apparatus must be periodically stopped to remove the particles before additional particles may be introduced into the bed.